In the fragment shader, we'll access our noise texture and if the value is below the threshold value Threshold, we'll use the reflected color (from the cube map), otherwise, we'll use a rust color:

in vec3 ReflectDir;
in vec2 TexCoord;

uniform samplerCube CubeMapTex;
uniform sampler2D NoiseTex;

uniform float ReflectFactor;
uniform vec4 MaterialColor;

layout( location = 0 ) out vec4 FragColor;

uniform float Threshold = 0.58;

void main() {
    // Access the noise texture
    float noise = texture( NoiseTex, TexCoord ).a;
    float scale = floor( noise + (1 - Threshold) );

    // Access the cube map texture
    vec3 cubeMapColor = texture(CubeMapTex, ReflectDir).rgb;

    // Gamma correct
    cubeMapColor = pow(cubeMapColor, vec3(1.0/2.2));

    vec3 rustColor = mix( MaterialColor.rgb, vec3(0.01), noise.a );

    FragColor = vec4( mix( cubeMapColor, rustColor, scale), 1);
}